This invention pertains to the art of temperature sensing cable apparatus and more particularly to temperature sensitive cable extrudates using polymers containing conductive and semi-conductive fillers. The invention is specifically applicable to continuous fire alarm cable systems for use with residential and commercial buildings or the like. It will be appreciated, though, that the invention has broader applications such as industrial manufacturing processes and other overheat detecting systems.
Fire alarm cabling and flame detecting devices typically include at least one pair of longitudinal electrical conductors impregnated within the cable itself. Thomas, in U.S. Pat. No. 2,483,793 discloses a fire alarm cable of the type broadly described above but further including short alternating readily combustible and non-combustible lengths of insulation. Low voltage current, derived from a step-down transformer, is provided at one node to a single common longitudinal conductor which extends throughout the length of the alarm cable. The other node of the step-down transformer is connected first to an audible alarm or bell and then, in turn, to a plurality of insulated longitudinal conductors which are made to lie in parallel with the common conductor for the length of the cable. Upon the occurrence of a fire, a portion of the cable is made to be consumed by the flames in order to melt and eventually sever the common conductor which in turn droops making contact with at least one of the plurality of parallel insulated wires. Such construction may be used to provide a crude estimation of an approximate location of the fire causing an alarm. However, the manufacture of such cable is difficult and the costs involved are accordingly high.
A flame and overheat detecting system is described in U.S. Pat. No. 2,941,192 to Postal. The cabling described there has an exterior protective metal sheath as of nickel-iron, a center longitudinal wire as of copper-clad nickel-iron, and a temperature-responsive electronic semiconductor filler material between the sheath and the longitudinal center wire. The filler material is essentially an insulator at low temperatures and a conductor at overheat or flame temperatures. The semi-conductive materials may comprise p-type semiconductors, including oxides of CO, Mn, Ni, Cr, and Cu, and the n-type semiconductors, including the oxides of Ti, Fe, or Ba, as well as mixtures thereof. Although such cabling has met with limited success, the complicated fabricating techniques involved, including the step of heating the cable to sinter the semi-conductive material for adequate electrical contact between the components and to reduce the oxygen content of the semiconductor to achieve the level of resistivity desired, has contributed to a higher overall cost.
Middleman, et al. in U.S. Pat. No. 4,352,083 discloses circuit protection devices having two columnar electrodes and a conductive polymer element, a part of which is a PTC element. The Middleman, et al. devices comprising PTC elements have been proposed to protect circuits against fault conditions arising from excessive temperatures and/or circuit currents. In general, electrical heaters comprising PTC elements used in this manner operate in a positive temperature coefficient range of electrical resistance. A material exhibits the positive temperature coefficient of electrical resistance when the electrical resistance of the material increases as the temperature of the material increases. The increase in temperature may be a result of either rise in ambient temperature surrounding the composition or by reason of resistive heating caused by the passage of electrical current through the composition. In this manner, current delivered to electronic circuits of the like may be regulated in order to protect those circuits against fault conditions. A material exhibits a negative temperature coefficient of electrical resistance when the electrical resistance of the material decreases as the temperature of the material increases.
The Middleman, et al. devices operate exclusively in the PTC region and are formed having constricted intermediate portions of relatively small cross-section to avoid the creation of localized "hot zones" as large proportions of the voltage drop of the PTC element nearly always takes place over a very small portion of the element.
Self-regulating electrically semi-conductive compositions, in the form of extruded flexible electrical cables, are often used in resistive heating, heat sensing, and circuit-breaking applications. For example, heating cables incorporating these compositions may be used for freeze protection of pipes and for maintenance of flow characteristics of viscous fluids in pipes and storage containers.
A popular class of self-regulating compositions which exhibit positive temperature coefficients of resistance are thermoplastic compositions comprising electrically conductive particles, such as carbon black, dispersed throughout a polymeric base. The resulting composition may be viewed as a polymeric matrix foundation within which is located an interconnected array of conductive channels formed from these carbon particles.
It has been theorized that the positive temperature coefficient of electrical resistance behavior of these compositions over a limited region of operation is caused by the expansion of the polymeric matrix at a rate which is greater than the rate of expansion of the conductive channels. The expansion of the polymeric matrix causes an increase, or other alteration, of the spacial relationship between the electrically conductive particles in a manner which causes the electrical resistance of the polymeric composition to increase. This increase in the electrical resistance (R) of the polymeric composition, for a fixed electrical potential (V) placed across the composition, causes the electrical current (I) passing through the composition to be reduced. Thus, the amount of heat generated by the passage of the electrical current through the resistive composition, given by relationship that heat (power dissipated) equals I.sup.2 R, or equivalently, V.sup.2 /R, is also reduced. Conversely, a decrease in the temperature of the matrix causes the matrix to contract, which places the conductive particles or channels in closer proximity to one another. This reduced spacing between conductive channels decreases the electrical resistance (R) of the polymeric composition which, in turn, causes the electrical current (I) to increase with a corresponding increase in heat generation.
An alternate theory, which does not depend on the expansion and contraction of the polymeric composition, explains the positive temperature coefficient of electrical resistance region of operation in terms of the amount of crystallinity present in the polymeric composition. According to this theory, the increase in the electrical resistance of the composition as the temperature of the composition increases may arise as a result of the reorientation of the crystalline-amorphic boundaries within the polymeric composition. This reorientation of the boundaries tends to electrically insulate the conductive particles (or groups of electrically conductive particles) from one another. The more effective insulation of the individual conductive components of the composition on the microscopic level contributes to the increase of electrical resistance of the composition on the macroscopic level.
Additional information on the general theory of how extruded semi-conductive polymers work may be in U.S. Pat. No. 4,200,973 entitled "METHOD OF MAKING SELF-TEMPERATURE REGULATING ELECTRICAL HEATING CABLE", issued to Farkas; U.S. Pat. No. 3,914,363 entitled "METHOD OF FORMlNG SELF-LIMITING CONDUCTIVE EXTRUDATES", issued to Bedard, et al.; U.S. Pat. No. 3,823,217 entitled "RESISTIVITY VARIANCE REDUCTION" issued to Kampe; and U.S. Pat. No. 5,057,673 to Farkas, et al, entitled "SELF-CURRENT-LIMITING DEVICES AND METHOD OF MAKING SAME".
The disclosure of each of the patents and all other materials referred to above is incorporated by reference herein.
Methods of making polymeric compositions of the type described above generally comprise a variety of process steps. The method steps often include: extruding the mixed compositions; applying shape retaining jackets to the compositions; annealing the compositions at or above their melt point temperatures; cross-linking the polymeric components with radiation; and, lastly annealing a second time. These steps, in a variety of combinations, are typical of procedures used in the production of semi-conductive polymeric compositions containing amount of carbon black ranging from less than about ten percent (10%) to greater than about seventy-five percent (75%) of the total weight of the composition. In most cases, the first annealing step takes up to twenty four hours after which the composition is radiated to cross-link the polymeric components. A post-irradiation annealing step may be selectively employed to further relax the polymeric matrix to achieve thermal stability and achieve electrical volume resistivities at room temperature in the range of from about five to one-hundred thousand ohm-cm. These steps, and the additional time required to perform the steps, further add costs to the end product. More importantly, the additional expenditures in performing the steps after extrusion are solely directed to improving the positive temperature coefficient characteristics of the polymeric compositions within which range the prior devices are confined to operate.
Accordingly, it has been deemed desirable to produce a cost-effective temperature sensing cable which is easy to manufacture without regard for good PTC characteristics, with an ability to identify an overheat event and its location, and operable in a region of NTC.